The cellular industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is outstripping system capacity. If this trend continues, the effects of rapid growth will soon be achieved in the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as to maintain high quality service and avoid rising prices. Furthermore, as the number of cellular users increases, the problems associated with co-channel interference become of increased importance.
Current digital cellular systems employ base stations which separate mobile signals using time and frequency orthogonality. Signals from a mobile propagate to a base station and the signals are received in a single or sometimes double antenna. The receiver processes the signal using time and frequency orthogonality to separate signals from different users. It is then possible to equalize and detect the signals. While techniques such as frequency hopping and advanced coding techniques provide ways for lowering co-channel interference, they are inherently limited by the available frequency spectrum. However, the use of the directional sensitivity of adaptive antennas offers a new way of reducing co-channel interference. An adaptive antenna consists of an array of spatially distributed antennas. Impinging on the array are signals from several transmitters. By properly combining the antenna outputs, it is possible to extract individual signals from the received superposition, even if they occupy the same frequency band. It is then possible to distinguish between spatially separated users by using narrow adaptive antenna lobes. This can be viewed as a way to utilize orthogonality in the spatial dimension.
Current digital cellular systems also employ base stations which use base antennas with wide antenna lobes, i.e., approximately 60.degree., 120.degree. or 360.degree.. The base station receives signals from all mobile stations within the lobe. It is hence not necessary to know the position of the mobile station. However, it is not possible to suppress mobiles transmitting from other angles. The use of narrow adaptive antenna lobes requires that the position or more exactly, the best spatial filters for reception/transmission to and from the mobile station be known. This implies that the spatial filters of the mobile must be measured for each new call and after each handover between base stations.
This measurement problem can be easily solved in many applications. However, the problem is much more important in cellular mobile applications where the mobile stations change position and where communication channels fade quickly. Furthermore, existing standards such as the GSM standard often assume that a wide antenna lobe is used so that valuable information can be sent directly to mobile stations with unknown positions. This implies that special care must be taken so that information is not lost during the training of the adaptive antennas. Another consideration is the linking to a channel, i.e., the fact that a mobile can be assigned to one of a number of time and/or frequency orthogonal channels. A new mobile may not be appropriate for a specific channel since, for example, it is close to an old mobile on the same channel. There is hence a desire to first measure, without disturbing any traffic, and then to link the mobile to an appropriate channel. In other words, one should maximize the spatial orthogonality.
Another important consideration is handover measurements. There is a desire to have some channels transmitted in a wide lobe so that the mobile station can measure the signal strength of the signal from the base stations.